New Hampshire's Manchester Union Leader, the only statewide daily newspaper in the leadoff presidential primary state, endorsed former Delaware governor Pierre S. (Pete) du Pont IV yesterday. Publisher Nackey Loeb acknowledged in her front-page editorial, "It is no secret that this newspaper has not been a flag-waving enthusiast for any of the candidates running for president. A lot of the conservatives are in the same boat." But, she said, "Of all those running, this newspaper believes that the best man for that job is Pete du Pont . . . . We are not worried about the man's courage or his commitment to the nation's tasks."

Loeb has made no secret that her preference would have been former U.N. ambassador Jeane J. Kirkpatrick, who decided against running. And Loeb notes that "we are happy" that du Pont, a Republican, has said he will include Kirkpatrick "as part of his team."

The endorsement will be a test of the Union Leader's political clout. Can the paper -- which has broken candidates in the past -- help make a candidate in 1988? Du Pont drew 3.5 percent support in a Union Leader poll released Monday. He is trailed in the GOP race only by Marion G. (Pat) Robertson at 3 percent and Alexander M. Haig Jr. with 1.5 percent. Vice President Bush led the field with 28 percent while his closest rival, Sen. Robert J. Dole (Kan.), had 10 percent. Rep. Jack Kemp (N.Y.), who had hoped his campaign would be boosted by the Union Leader, was third with 8 percent.